Preheat the oven to 350°F (175°C). Butter a 9-inch springform pan or an 8-9 inch round cake pan and line the bottom with parchment; butter the parchment as well.

Melt chocolate and butter

Chop the chocolate into small pieces for even melting. In a heatproof bowl set over (not touching) simmering water (double boiler), combine the chopped chocolate and butter. Stir gently until completely melted and smooth, then remove from heat and let cool slightly (about 5 minutes).

Dissolve espresso

While the chocolate is cooling, dissolve the instant espresso powder in 1 tablespoon of very hot water; stir until fully dissolved. Add this espresso to the melted chocolate and butter and stir to combine.

Whisk eggs and sugar

In a large bowl, whisk the eggs and granulated sugar vigorously until mixture is pale and slightly thickened (about 2-3 minutes by hand or 1-2 minutes with an electric mixer on medium). The mixture should be homogenous but not aerated like a sponge cake batter — a glossy ribbon is fine.

Combine chocolate and eggs

Pour the slightly cooled chocolate-espresso mixture into the egg-sugar mixture in a steady stream, whisking constantly to temper the eggs and ensure a smooth batter.

Add dry flavorings

Sprinkle in the flour, ground cinnamon, and salt over the combined batter and fold gently with a spatula until just incorporated. Fold in the lemon zest until evenly distributed. The batter will be fairly dense and glossy.

Bake

Pour the batter into the prepared pan, smoothing the top with a spatula. Bake in the preheated oven for 25-30 minutes for a slightly fudgy center, or up to 35 minutes if you prefer it more set. The center should still have a slight wobble but the edges will be set. Do not overbake.

Cool and release

Remove the torte from the oven and set the pan on a wire rack. Allow to cool completely in the pan (at least 1 hour) to set. Once cool, run a knife around the edge and release the springform (or invert if using a regular pan) onto a serving plate.

Garnish & Serve

Dust the top lightly with unsweetened cocoa powder using a fine sieve. Scatter chocolate shavings over the center and sprinkle the additional lemon zest to highlight the citrus note.

Serve slices with a spoonful of whipped cream or mascarpone alongside. The torte can be made a day ahead and kept covered at room temperature for up to 24 hours or refrigerated for up to 3 days (bring to room temperature before serving for best texture).